The present invention concerns a method and apparatus for the extrusion of plastic pipes with composite walls.
Plastic pipes are currently being used pipelines or pipe systems such as water mains, sewer lines, conduits and the like because of their light weight, their ease of placement or positioning and their ease of linking or connection with each other as well as their resistance to corrosion. It is now customary for these pipes to be a composite structure formed by three layers. Two of the layers form the exterior and interior walls with the intermediate or third layer or core being either of identical or different material from that forming the walls. Very often the third layer is made of lighter material than the other two. Such three layer walls are customarily made by co-extrusion.
Equipment and a performing process making it possible to construct such composite pipes are described in French patent No. 2,455,972. The equipment according to that patent is constituted of a distribution head, which is heated and regulated and which is supplied with a molten plastic material by a first extruder. The distribution head uses the material from the first extruder to form the exterior and interior walls of the pipe and uses a second molten plastic material from a second extruder to form the core of the pipe. This distribution head possesses channels to separate the first molten material into two distinct streams of flow to form the two walls. Between these two streams there exists a channel for introduction of the second molten material constituting the core as a third stream. Before passing into the die which forms the composite pipe, the three united streams are passed over a divergent cone arranged at the outlet of the distribution head where the three streams of flowing material are united or converged into a single stream.
According to the prior art, one of the extruders supplies the first material to the distribution head in standard axisymmetrical fashion. Such a supply requires the use of a first set of blades mounted in the flow path of the molten material in order to support the metallic core forming part of the distribution head. In the same way, at its greatest diameter, the divergent cone requires the existence of a second set of supporting blades making it possible to attach the divergent cone to the apparatus in a rigid spaced relationship. In this type of equipment, a first set of at least three blades support the distribution head center section or metallic core and a second set of at least four blades support the divergent outlet cone. These two sets of blades are located in the path of flow of the molten plastic materials and are particularly troublesome in that, first, they leave more or less rectilinear traces on the final material and, second, in order to reduce or eliminate these traces by allowing the cleanest possible fusing of the molten products after passing the blades, it is necessary to have considerable separation between the last blades and the die thereby requiring the use of an apparatus of extended length.
In order to obtain multi-layer films, a complex system has been proposed in U.S. Pat. No. 3,576,758 consisting of axiasymmetrically parallel ducts ending at different distributors for each of the flows located ahead of the common die as in the case of the divergent cone of the preceding technique. These flow distributors are located or positioned in a direction perpendicular to the axis of the equipment and necessarily comprise a set of blades for support and have the disadvantages previously disclosed. With that technique of axiasymmetrical material supply, the first set of blades of the distribution head have been eliminated, but it is not possible with that system to eliminate the other set of blades preceding the die.
From this known prior art, it is not possible to construct multi-layer pipes by co-extrusion of materials without using apparatus comprising at least one set of blades mounted in the flow path of the molten materials.
The equipment according to the present invention overcomes these prior art disadvantages by the fact that all of the supporting blades are eliminated. The integrally formed distribution head and mandrel are formed as a part of the body of an extruder up stream of the supply of the molten material to the flow distributors. This improvement makes it possible to substantially improve the surface appearance of the pipes produced.
The invention comprises a novel distribution head that is heated and regulated. It is in operative relationship with a first extruder which supplies the molten plastic material constituting the walls of the pipe and with a second extruder which supplies the molten plastic material constituting the core. The distribution head in a novel manner possesses a means of separating the flow of the molten plastic material which will constitute the pipe exterior and interior walls into two distinct streams of flow. Further, it includes a novel means of introduction of the molten plastic material constituting the core which forms a third stream of flow between the first two streams of flow and a means of bringing the three streams together and placing them in contact with each other in order to form the rough shape of molten material which is supplied to the die and mandrel forming the extrusion tool.
This equipment is characterized by the fact that the means for supplying the two separate streams of molten material to form the walls and the means of introduction of the molten material constituting the core are arranged axiasymmetrically in relation to the axis of the distribution head and extrusion tool in such a way that a unitary device is formed and fixed directly to the body of an extruder up stream of the supply of the molten flow of material to the distribution head.
The unitary device has a central body portion and first, second and third concentric cylinders of successively increasing diameter encircling said central body portion and each other in spaced relationship to form three concentric annular spaces for receiving molten material. Only one end of each of the first, second and third cylinders and the body portion are attached to each other so as to maintain the rigid spaced relationship. A mandrel is attached to the outer end of the central body portion and a die is attached to the outer end of the outermost concentric cylinder in spaced relationship with the mandrel to shape the molten material from the three annular spaces into a pipe having a multi-layered wall.